1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus in which ultrasonic waves are transmitted and received in directions each along the associated one of a plurality of scan lines extending in a depth direction within the subject to obtain received signals, and data representative of displacement of the respective pixel points along each scan line within the subject are evaluated on the basis of the thus obtained received signals, thereby displaying an image based on the data representative of the displacement.
2. Description of the Related Art
Hitherto, there has been used an ultrasonic diagnostic apparatus in which ultrasonic waves are transmitted toward the subject, specially a living body, ultrasonic waves reflecting from a tissue within the living body are received to generate received signals, and a tomographic image of the living body is displayed on the basis of the received signals, thereby facilitating a diagnostic of diseases of the viscus inner organ or the like in the living body. According to such an ultrasonic diagnostic apparatus, usually, there is also provided such a function that a blood flow velocity within the subject is evaluated on the basis of Doppler displacement information carried by the received signal obtained through a plurality of number of times of receiving in the same direction within the subject, the blood flow of which a direction approaches a probe is indicated by, for example, red and the blood flow of which a direction is going away from the probe is indicated by, for example, blue, and a color image representative of the blood flow velocity with a luminance of those colors is created.
The above-mentioned tomographic image is referred to as a B-mode image and is usually displayed with a black-and-white image. Thus, image data representative of the B-mode image is referred to as black-and-white data. On the other hand, an image representative of a blood flow distribution is usually superposed on, for example, the B-mode image and displayed with color. Thus, the image representative of the blood flow distribution is referred to as a color Doppler image or a color image. And image data representative of the color Doppler image is referred to as color data.
FIG. 5 is a block diagram showing a schematic construction of a portion of a color Doppler function of an ultrasonic diagnostic apparatus according to the related art.
High voltage pulses are applied from a transmitting and receiving unit 2 to an ultrasonic probe 1. Upon receipt of the high voltage pulses, the ultrasonic probe 1 transmits ultrasonic waves into the subject (not illustrated). The ultrasonic waves transmitted from the ultrasonic probe 1 are subjected to a Doppler displacement with the blood flow within the subject and reflected into the ultrasonic probe 1. The ultrasonic waves thus received by the ultrasonic probe 1 are converted into received signals. The received signals are fed to the transmitting and receiving unit 2 so as to be subjected to a predetermined beamforming process. An output of the transmitting and receiving unit 2 is fed to a quadrature detector 3 so as to be subjected to a quadrature detecting process. A Doppler displacement component obtained through the quadrature detecting process by the quadrature detector 3 is supplied to an A/D converter unit 4. Digital signals obtained through the A/D converter unit 4 are fed to an MTI filter 5 to remove a clutter component and then fed to a velocity arithmetic unit 6 to evaluate a velocity of a blood flow on each portion within the tomographic plane.
The blood flow velocity obtained through the velocity arithmetic unit 6 includes a remarkably large noise component. Consequently, the signals passing through the MTI filter 5 are also fed to a power arithmetic unit 7 to evaluate a blood flow power. The blood flow power thus obtained is supplied to a first noise suppression filter 8 in which the blood flow velocity evaluated in the velocity arithmetic unit 6 is replaced by the velocity value zero so that no color display is implemented with respect to a point or an area on which the blood flow power is given by a value less than a predetermined level. The blood flow velocity outputted from the first noise suppression filter 8 is fed to a scan converter 9 to be converted into data for a display, and further passed through a second noise suppression filter 11 to a CRT. Upon superposing the blood flow velocity thus processed on the B-mode image based on, for example, black-and-white data, a color Doppler image is displayed on screen of the CRT.
According to the second noise suppression filter 11, the blood flow velocity is replaced by the velocity value zero so that no color display is implemented with respect to pixels involved in black-and-white data associated with ones exceeding a predetermined luminance, in view of the fact that on the B-mode image the inside of the blood vessel is displayed with the lower luminance, and the walls of the blood vessel and the domains of a high density of tissue are displayed with the higher luminance.
Incidentally, while the black-and-white data are also generated on the basis of the received signals outputted from the transmitting and receiving unit 2, the way of generation of the black-and-white data is not directly concerned with the present invention ans is also well known. Hence, the explanation of such a technology will be omitted. Further, the fundamental arithmetic technology for obtaining the color data is also well known and thus additional explanation will be omitted.
FIG. 6 is a typical illustration of color noises on an image.
According to the ultrasonic diagnostic apparatus of the related art as mentioned above, since the blood flow velocity obtained through the velocity arithmetic unit 6 includes large noises, the noise eliminating processing is performed on the basis of the blood flow power by the first noise suppression filter 8, and further additional noise eliminating processing is performed on the basis of the value of the black-and-white data (luminance of the B-mode image) by the second noise suppression filter 11. However, the blood flow power and the noise power do not always match, and further it does not always say that portions other than the blood flow display portion involve high luminance. Thus, the ultrasonic diagnostic apparatus according to the related art has been associated with such a drawback that color noises remain thereby providing images which are hard to see on each frame.
Specifically, when the blood flow on a deep portion within the subject or a low velocity of blood flow is displayed on a display screen, if a gain of the color Doppler image decreased until the color noises disappear, the sensitivity also goes down and as a result it happens that not only the noises but also the essential blood flow are not clearly displayed.